1. Field of the Invention
The present invention relates to a dielectric waveguide suitable for a transmission line or an integrated circuit used in a millimeter wave band or a microwave band.
2. Description of the Related Art
A dielectric waveguide having a dielectric strip between opposing parallel conductors has been used as a transmission line used in a millimeter wave band or a microwave band. In particular, a dielectric waveguide in which the distance between the conductors is set to a value smaller than 1/2 of the wavelength of propagating electromagnetic waves to limit radiated waves at a bent portion of a dielectric strip has been used as a nonradiative dielectric waveguide.
Dielectric waveguides of this kind may be used to form millimeter wave circuit modules and may be connected to each other between the modules. In such a case, dielectric strips are connected to each other. Also, if dielectric strip portions are not integrally formed in a single module, dielectric strips are connected to each other.
FIG. 35 shows a conventional connection between two dielectric strips. Upper and lower electrodes are omitted. Members 1 and 2 are dielectric strips. Dielectric waveguides are connected to each other by opposing the end surfaces of the dielectric strips which are perpendicular to the direction of propagation of electromagnetic waves.
Conventionally, polytetrafluoroethylene (PTFE), which has a small dielectric constant and exhibits a low-transmission loss, has been used to make a dielectric strip, and hard aluminum having high workability and having a suitable high hardness has been used as a material for forming an electroconductive plate constituting a dielectric waveguide. However, the difference between the linear expansion coefficients of PTFE and aluminum is so large that a gap is formed between the opposed surfaces of dielectric strips of a dielectric waveguide when the dielectric waveguide is used at a temperature lower than the temperature at the time of assembly. Ordinarily, a certain gap can also exist between the opposed surfaces of dielectric strips according to a working tolerance. Since the dielectric constant of air entering such a gap is different from that of the dielectric strips, reflection of an electromagnetic wave occurs at the gap, resulting in a deterioration in the characteristics of the transmission line. Moreover, at the time of assembly of separate dielectric waveguides, a misalignment may occur between the opposed surfaces of the dielectric strips at the connection between the two dielectric waveguides, which depends upon the assembly accuracy. In such a case, reflection is caused at the connection surfaces, also resulting in a deterioration in the characteristics of the transmission line.
FIG. 36 shows the result of calculation of an S11 (reflection loss) characteristic in a 60 GHz band of a dielectric waveguide which has a sectional configuration such as shown in FIG. 1, and in which, referring to FIGS. 1 and 35, a=2.2 mm, b=1.8 mm, 2=0.5 mm, gap=0.2 mm, LL=10 mm, and the dielectric constant .epsilon.r of 2.04. The characteristic was calculated by a three-dimensional finite element method. The guide wavelength .lambda.g at 60 GHz in this case is 8.7 mm. As shown in FIG. 36, even when the gap is small, about 0.2 mm, the reflection loss is -15 dB or larger.